Virtually all Western marriages are preceded by a series of relationship transitions, beginning with movement into new romances (i.e., falling in love) and followed by a series of changes in the depth of association (e.g., increasing commitment versus breaking up). Despite the ubiquitousness of nonmarital romantic relationship transitions, and the powerful role these bonds more generally play in individuals'lives, scientific knowledge regarding how such transitions shape the physical and mental health of those involved is sparse. The overall objective of this application, which represents an important first step in this endeavor, is to determine how transitioning into and out of nonmarital romances affects never-married emerging adults'acute stress reactivity, physical health, and mental health outcomes as a function of the qualitative nature of the transition. The central hypothesis of this application is that nonmarital relationship transitions differentially affect individuals'physical and mental health outcomes. This hypothesis will be tested by pursuing three specific aims: (1) To examine the effect of transitions into and out of nonmarital romances on individuals'acute stress reactivity, physical health, and mental health;(2) To assess the extent to which dependence on a relationship moderates the effects of relationship termination;(3) To assess the extent to which gender moderates the effects of each transition. Never-married emerging adults in the early developmental phase of their nonmarital romances will be exposed to a standardized acute stress task during either the beginning of their relationship, after it ends, or after the relationship has persevered for at least 9 months. Cortisol responses will be assessed during exposure to the acute stress task. Additionally, participants will provide self-assessments of their current physical and mental health at the start of the study and after their relationships has persevered or terminated. The proposed work is innovative because it highlights the physical and mental impact of transitions into and out of nonmarital romances in a single investigation. The work is significant because it expands the scientific study of close relationships and their health impacts and could serve as the foundation for intervention strategies designed to help individuals better understand the impact their romantic relationship experiences have on their health. PUBLIC HEALTH RELEVANCE: Given the prevalence of involvement in nonmarital romantic relationships for individuals across the lifespan, this is an important topic that has the potential to significantly enhance knowledge of how nonmarital romances affect individuals'physical and mental health outcomes.